Minimally invasive treatments such as Laser-Induced Thermotherapy (LITT), Radiofrequency Ablation (RFA), High Intensity Focused Ultrasound (HIFU) and microwave ablation are commonly used in the clinical setting for the treatment of focal cancers throughout the body. Magnetic Resonance Imaging (MRI) based temperature monitoring is often performed in order to provide real-time feedback to the operating physician, typically using the Proton-Resonance-Frequency (PRF). Another commonly used minimally invasive local tumor therapy is cryoablation, which creates an ice ball to induce cell death. As for high temperature thermal therapies, it is often necessary to monitor temperature changes in real-time during a cryoablation procedure, particularly in at-risk structures adjacent to the target tissue/organ. Currently, temperature monitoring is performed using invasive temperature probes which must be placed by the operator, a time consuming and potentially dangerous procedure. Considerable research was made for non-invasively measuring the sub-zero temperatures within the ice-ball itself using MR. See “MRI-Guided Cryoablation: In Vivo Assessment of Focal Canine Prostate Cryolesions”, Josan S, Bouley, Van den Bosch, M., Daniel B L, Butts Pauly K. J Magn Reson Imaging, 2009 Jul. 30 (1): pages 169-176. See also “A Fast Calculation Method for Magnetic Field Inhomogeneity due to an Arbitrary Distribution of Bulk Susceptibility”, Salomir et al, Concepts in Magnetic Resonance Part B, Vol. 19B, pages 26-34 (2003).
However, there has been little or no investigation into using MR to measure the near-zero temperatures which are induced around the ice ball. As long as the tissue still contains liquid water, the PRF method should be applicable. However, the ice ball 10 (see FIGS. 1A, 1B) itself disturbs the local magnetic field because of susceptibility contrast with defrosted tissue, which strongly influences the PRF method.
Thus, between the frozen tissue and the not-frozen tissue a difference in susceptibility exists which leads to a distortion of the local magnetic field 11 as shown in the prior art FIG. 1A. This distortion of the local magnetic field in MR temperature-imaging causes an over- or underestimation of the temperature, as shown in FIG. 1B at 8 and 9.
The advantages and disadvantages of cryoblation are thus the following.
The advantages are:
1) much less painful than “burning” techniques—LITT and RFA:
2) the ice ball can be clearly visualized; and
3) the ice ball can be sculpted using multiple probes—to contour complex or large lesions.
The disadvantages are:
1) bleeding (during and after thaw)
2) low signal in the ice ball—research has been done by the group of Kim, Butts, Pauly; and
3) susceptibility artifacts in the MRI phase data around the ice ball—temperature error of ±10° C. for ice ball diameter of 2 cm—important for organs of risk around the ice ball.
See also, Josean S et al. MRI-Guided Cryoablation: “In Vivo Assessment of Focal Canine Prostate Cryolesions.” J. Magn. Reson. Imaging 2009:30, pages 169-176.
Thus, the ice ball 10 (see prior art FIGS. 1A, 1B) itself disturbs the local magnetic field “because of susceptibility contrast with the defrosted tissue. As a result, the commonly used Proton Resonance Frequency (PRF) method under- or over-estimates near-zero temperatures as shown at 8 and 9 in FIG. 1B.